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Chapter XXVIII. The Sense-Organs

In this chapter we have to consider n large number of structures. Concerning the lower sense-organs, touch, taste, etc., wo know almost nothing; concerning the olfactory organ a little, concerning the eye and eur a good deal — on the embryological side, of course.. We have further to emphasize those traces which have been discovered of long series of sense-organs, of which the nose, eye, and ear are probably derivatives, in the ancestors of vertebrates, although in all known vertebrates most of these series have become rudimentary or lost. The serial sense-organs I designate under the comprehensive name of ganglionic sense-organs. There are probably two, and only two, series along each side of tlu^ body : one series, the upper, corresiX)nds to the lateral lino of I'omparative anatomy, the other to the epibranchial lin(\ The olfactory, visual, and auditor^' organs are i)robably specialized gjmglionic sense-organs. The organs of touch, taste, etc., have not yet been shown to have any genetic relationship to the ganglionic sense-organs.

Ganglionic Sense-Organs

By this term I propose to designate the series of organs formed by the temporary or permanent union of ihi) sensory ganglia, described in the previous chapter, with the epidermis. The discovery that such a class of organs exists, and that the ear ])robably, the eye and nose possibly, Mong to the class, was due to Froriep, 85. 1, whose article marks an important step in vertebrate morpholog}'. The temporary coimection of certain ganglia with the epidermis was, so far as I know, first discovered by J. W. Van Wijlie, 82. 1, in elasmobranch embryos, and has been esj)ecialh' studied by Beard, 85.1, whose researches have proved valuable and suggestive, though his publications are marred by premature and too diagrammatic generaliziitions. Beard proposes the name of branchial sense-ofyaiis, but the term most generall}' useil is segmental sfuse-orrfans, because the organs are believed to be repeateil in each segment. The term adopted here, '* ganglionic," is purely descriptive, and involves no theory as do the two others just mentioned, and moreover serves to indicate also the distinction between the two main classes of sense-organs.

As an example of a typical ganglionic sense-organ of an embryo we may take the front ganglion of a young elasmobranch. This ganglion belongs to the third cranial or (X*ulo-motor nerve. According to Tk^ard it grows out from the neural crest of the mid-brain shortly bef<ire the closure of the medullary groove at that point. It soon comes in contact with the epidermis, which thickens where the ganglion touches it. the thickened epidBrmiu and tho gmiglion tiiuii

Additionjil ih'tails aiv givcu iu counectioD with the history of the cephalic nerves in the first edn. chapter, p. 033.

futte. Fig. 4(lti, the former hecominK uepreH8ed ho hh to mulco a hIihI
low pit. The boundary Itetweeii the two tifHiieH tieconic'H iiidiHtiiK^.

According to Beard some of the cells are specialized later to form what he terms the BUjini
branchial nerve, and of this his figum, Fig, 40(1, indicates the commencement, tip.n. tijiniilar fusions have been demonstrated in the amniota by Froriep in the ca»o of the ganglia of the facial, glosso-pharyngeal, and vagus nervtm; the site of the fusion is for each of these ganglia directly alwve the gill-(rleft to which the nerve of the ganglion bel<>ngs. The term brancliiul sense-organ has referenceto this position, which is assumed by Beard to be typical for all the organs of the class.

0. JCupffer, 91.1, has Hhown that in I'etromyzon the ganglia frjrm two series of unionti with the epidermis, and maintains that at <!furh , point of union cells are Iniddeiloff from the epidermis and incorporated in the ganglion which is so enlarged. Fig, 41)7 illustrat<M the ar- aiaPA!"'Mv-'r S' rangement of the ganglia as found by him in a young Petromyzon ( Amm'xxetes of 4 mm.). thefivegniatganKlin of the head (the cnliarj-, 1.; trigt^minul, II.; w.-(mstiu>-ff»;ial. 111. ; gloaso-pharjTigeal, IV. ; and vn(fii«, V.j are each iiiuutn^Hl with the epidermis and receive cells from it; the line of the ganglia is pr*ilonged backward by the lateral line; if the line of l\ii- ganglia were continued forward it would, allowing f'lr tin- l»end of the Ixwl, UfTminate in the nasal pit, -V, or anlage of tin; olfa<ft'iry organ ; the <Mr (otocyst) lies directb' along the line of the gjingUa, and r<;proHeiitM,

a. hatf l/een long known, an epidermal area in r.-onta/.-t with tli« acoustic ganglion: tbe olfactory* area, an vi'i b<ive seffTi, \>. /A"!, aW) throws off celbs and produces a ganglion, whi^rh we may, hy[x/thet> ically at leaiit. add to the ckaio of ganglia I.-V. We hitve tt*>^i iIk;

lateral line prolonged forward by the five primary cephalic gan^Iia^ and by the olfactory ganglion the entire length of the head. It is, I think, not carrying speculation too far to suggest that the retina of the eye represents another ganglionic area, which has been disphiced by being involved in the invagination to form the foi'e-brain ; it is perhaps not superliuous to add that the acceptance of this speculation encounters difficulties which cannot, at present, be removed. Each of the primary ganglia. Fig. 407, I.-V., is prolonged downward and joins a chain of epibranchial ganglia, which are connected together longitudinally with one another and are much smaller than the main or lateral ganglia; every epibranchial ganglion, also, is connected with the epidermis and receives cells from it; the epibranchial chain begins immediately beliind the lens of the eye and is continued far backward above the mouth and then above the gill-clefts. Kuj>flfer figures twelve epibranchial ganglion ; the first is immediately behind the lens of the eye and is united with the ciliary ganglion, I. ; the second, third, and fourth overlie the mouth, the third being connected with the trigeminal ganglion, II. ; the fifth to twelfth ganglia lie each above a gill-cleft, there being eight gill-clefts present at this stage ; the fourth and fifth appear to be connected with the acousticofacial Iffteral ganglion. III. , the sixth with the glosso-pharyngeal, IV., the seventh with the vagus ganglion, V.

As Kupffer points out, /.c, 41», we have to do in Petromyzon with very primitive conditions, which must contribute much toward the comprehension of the morpholog}^ of the ganglia and sense-organs of the higher vertebrates. There can bo little doubt that the lateral ganglia and sense-organs as one series, and the epibranchial ganglia and sense-organs as another series, are common to all vertebrates. As already stated, it seems certain that the ear, probable that the olfactory organ, and possible that the eye all belong to the lateral series, and there can be little doubt that the organs discovered by Froriep, and ni^w generally known as branchial sense-organs, are members of the epibranchial series. I deem it extremely probable that further investigation will demonstrate the existence of both series in the embryos of all vertebrates.

There is little in the embryonic organs described, beyond the union of nervous and epidermal tissue, to suggest comparison with a histologically specialized sensory apparatus; nevertheless we may safely interpret b<:>th the lateral and epibranchial structures as rudimentary sense-organs, l)ecause in the case of the ear and nose, as described below, such a union constitutes an essential stage in the development, and because the fact that the organs of other ganglia abort during embryonic life accounts for the lack of the histological differentiation. The number and fate of the rudimentary ganglionic senseorgans has been discussed in connection with the history of the separate cranial nerves. The problem of the homologies of the organs with sense-organs of invertebrates is still too obscure to be profitably discussed here.

Very suggestive in this connection are the observations of H. V. Wilson, 91.1, 244-253, of a thickening of the nervous layer of the epidennis on either side of the head in the bass embiyo (Serranus atrarius) . This thickening forms a long, shallow furrow, which subsequently divides into three parts, of which the first becomes a sense-organ over the gill-cleft, the second the auditor}^ invagination, and the third, the anlage of the sense-organs of the lateral line. This peculiar development confirms the notion that all these organs belong in one series, but the appearance of a continuous thickening as the anlage of them all has, as yet, been observed only in this fish, and may not indicate a corresponding ancestral condition. Unfortunately Wilson was unable to make out anything as to the connection of the sensory plate with the ganglia. The sense-organ above the gill-cleft, though differentiated, is a larval structure only, and disappears in the adult.

Evolution of the Ganglionic Sense-Organs

Lenhossek, 92,1, has shown that in the earth- woi-m there are cells scattered through the epidermis which give off fibres which run to the central nervous system, and there like vertebrate sensorj^ fibres fork ; one fork runs hemiward, the other tailward within the central ganglionic chain. This important discovery renders it probable that sensory ganglion cells and sensory cells were originally one, and that the ganglion cells of vertebrates are nerve-sense ceDs, which have migrated from the epidermis. The ganglionic senseorgans in this way are traced to a genetic condition arrested, for we may assume that they correspond to areas in which the nerve-sense cells are congregated, and that part of the cells remain in the epidermis, while others migrate from it to constitute the ganglion. Lenhossek's discover^' leads him to the further hypothesis that the si)ecial sense-cells connected with a nerve-fibre, such as occur in taste-bulbs, the olfactory membrane, and the organ of Corti, are really comparable to the nerve-sense cells of Lumbricus, and are true neuroblasts in that they produce the nerve-fibres connected with them; hitherto we have assumed that the nerve-fibre grew to the cell. It seems to me that Lenhossek's hypothesis is likely to be verified with revolutionary results for our conceptions of the morphology of the nervous system and sense-organs.

The Special Sense-Cells

I wish to point out that there is a remarkable uniformity in the specialization of the sense-cells in the organs of taste, smell, sight, and hearing, which at once suggests that they are all derived from a common form. The cells are elongated and have, 1, a lower tapering infra-nuclear member, which is a portion of the protoplasmatic body of the cell, and is, probably, always connected with a nerv'e-fibril ; 2, an upper supra-nuclear member, which is also part of the protoplasmatic cell ])ody and stretches to the surface of the epithelial layer in which the 8|)ecial sense-colls are situated ; 3, a projection above the surface of the epithelium ; the projection is different in character from the protoplasm ; it differs also according to the organ ; the projection is called a hair or cilium in the case of the organs of taste, smell, and hearing, a rod or cone in the case of the eye.

The obvious similarity of the special sense-cells confirms, I believe, the theory that the special sense-organs are mollifications of ganglionic sense-organs, which in the ancestors of vertebrates were all similar and perhaps serv^eil a generalized sensory function . Perhaps the sensecells are also nerve-sense cells, as suggested by Lenhossek (above).

Organs of Touch and Taste. — 1 havo Invn iinablo to fiml a single w<.»nl as U) tho ilovolopinont of any taotile organs by any writer.

Olfactory Membrane

The development of tho nasal pits and their onlargiMnont to form tho nasjd oavity is di^sorilnnl, p. 57r»; tlu^ dovolopmont of tho olfactory ganglion and norvo from olfactory opitholium is dosorilxnl, p. (»:>7. Conooniing tho further history of tho olfactory mombram* and tho gtniosis of its sonst»-(*i'lls nothing is known.

It may Ih» rtvallinl that Rlant\ 84.1, has rtM*ordtHl that in various tishos tho olfactory tn^Hs aiv ci>llocttHl in groups, haviug a vi»rv striking similarity to InUh tho tasto-bullw and tho sonso-i>rgans of tho lateral lino of ananmiota. In mannnals tho olfactorv tvlls an* not so grouiHHl, but it is [K^ssiblo thoy may 1m» so in tho ombryo. As tho organs of tho lateral lino aro ganglionic st»nso-organs, Blauo's i>l)sorvatii>ns otTor additional ovidoniv (or intorpn^ting tho organ i»f smoll as likewise a ganglitniic sonst*-i>rgan.

The Optic Vesicles

The first stages of the optic vesicles nn divert i(*ula of the foiv-brain have Invn tractnl alK>ve, p. r)t»4. The vesicles form the n»tina, the chon»id c<vit, and tho o]>tic nerve (»f the atlult eye ; the ditYorentiation of the anlag«*s of thost* thixH* |>arts forni.s tho subject of this section.

Tho ftUUnving account of the early changes in the shaiK? of the optic vesicles in the human embryo is U-i.^ed on His, 89.4, <IS5, wlu> has also traivd, 68.1, UU/i:V2, and 74.1, 1(K>, tho ct)rrt*s|H»iuling changi^s in the chick. The vosicliv; Wcomo .stalktnl by the fourth wivk; the stalk. Fig. .'5:^7, springs from tho lower e<lgo of tho fon^-brain ^ halamemx^phalon) just in front of tho infundibular region; the base is bn»ad, but very rapidly tajiors down to the narrow stalk proper; the end of tho vesicle is onhirgoil and the enlargement ex]>iuids upwanl and backwanl, as in all vertebrates. The out.T and l,>wer ]>osterior wall of the vesicle and part of the iKTsterior wall o( tlio stalk Invome puslunl in, and thon^bv tho vesicle is ohangtHl int> tlio so-calli^ "optic cup." Tho invagination is pn)l)ably duo in mai;. as in tho chick (Foster and Balfour, "Elements,'" "id ed., p. r»0 and in th.^ rabbit (as I have observinl), to the contact of the distal wall of the optic vesicle with the overlying epidermis: where the (n>nta<"t occui*s the wall of the vt^icle and the epidermis Ix^comes ap|\ariMitly cU>st»ly unittnl, as if glued together; the union takes place in the chick the end of the second day, in the rabbit the end of the ninth; over thn aroii of tlin union liotli ltiji>m iHU'imin Miickcnixl,

bonlei- of the fisBiire ie the Seiteiiieisie of His, the lower border the Busifarleiste ot Hia; the fissure is kuown as the choroid fissure ; it is occupied by meaenchj'ma ; and there is developed, probably, eai-ly in the fifth week in man, a blood-vessel, which runs along the furrow to branch out in the retinal cup between the retina and the lens; this vessel is the arteria ceniralis reiintB or arteria hifaloidea. During the lifth week the choroid fissure begins to close; the closure commences at the proximal end and progresses toward the retinal end of the stalk ; a little later the fissure closes at the lower edge of the retina; there is thus left. Fig. 4Ht, Cli.f, a short stretch of the fissure open. It is through this opening that the arierhi centralis. Art, enters and jtasses on to the hollow of the retinal cup; it is prolonge<l through the vitreous humor, and there breaks up into numerous branches, which run toward the posterior surface of the lens, where the terminal branches spi-ead out to prodiice the tunica vasculosrt enveloping the lens. In the human embryo at three months the central artery gives off a cone of branches with no main stem (or arteria liyaluidea proper) which run through the vitreous humor to the lens; and at this age the atrophy of the vessels has begun (O. Schultze, lsi»3. in " Festschrift zum SOiiihr. Duktorjub. von Killliker"). At five to six months most of the branches have aborted, and the main hyaloid trunk is developed as a continuation of the arteria centralis through the vitreous humor. During the last month of foetal life the vessels of the vitreous humor abort completely, and the only trace of their existence to be preserved is the canalis j, ^

htjaloideuSy which corresponds to the space originally occupied by the main stem of the artery. With the disappearance of the artery the last remnant of the choroid fissure closes. The secondary optic

walled cup formed by the invaginated retina and the pigment membrane covering it. The o})ening of this cup is closed by the lens, and so remains throughout life. As seen in Fig. 409, the lens at first nearly fills the cup, but as the retina and pigment layer grow rapidly, the optic cup enlarges and becomes the anlage of the ball of the eye, and the space between the lens and the retina is increased until it assumes the adult dimensions ; the space is occupieil by loose immigrant mesenchyma, which forms the anlage of the vitreous humor. As the eye expands the tissue around it is condensed and forms an envelope of connective tissue inclosing the optic cup or eyeball ; the envelope is the anlage of the sclera and choroid. I regard it as probable that the condensation is a mechanical result of the expansion of the optic cup.

The position of the eye is at first lateral, with the axis turned slightly forward; in the course of its further development (His, 89.4, 689) it moves more and more from its original site downward and forward. Until the end of the first month it lies near the side of the thalamencephalon and higher up than the infimdibular process. During the fifth week it gradually descends from this level, and later swings around more and more toward the front, and by the end of the second month it lies below the olfactory lobe. During the latter half of the second month the two eyes have their axes at an angle to one another of about 90 degrees ; during the second month the angle further diminishes, and ultimately — the exact time is not known — the axes become parallel with one another. The insertion of the optic stalk is from the start eccentric; at first it is on the lower side of the optic cup, but as the eye migrates it comes to lie <^n the inner side of the eyeball ; it remains eccentric throughout life. At no time does the insertion of the stalk (optic nerve) coincide with the position of the macula lutea, as luis been erroneously assumed.

By referring to Fig. 409, it will be seen that the edge of the optic cup lies on the outer surface of the lens ; Jtliis is always the case. The orifice of the cup is the iuture pupil of the eye; it is a circular opening through which the surf ace of the lens is exposed. As the eye grows the lens enlarges, but the orifice of the cup (or pupil) does not become larger;* hence there annes to be a portion of the optic cup which rests on the anterior surface of the lent*. Fig. ■ill, L'v. In the region of the optic cup around the e<lg« of the lens and on the front surface of the lens, both layers (retinal, R, and pigment, p) of the cup liecoine thin and very closely unit«<:l, so that from an early stage their development progresses as if the}' were one layer; a short distance frtim the lens the retinal layer thickens and extends over the rest of the optic cup as the aniago of the true retina. The thin-walled jwrtion of the optic c»ip maj- be called the lenticular zone; the portion of the zone aiwtnd the pupil and resting on the lena forme the double epithelial pigment layer of the adult iris and might be appropriately designated an the jirimitive ins; the portion of the zone aroimd the lens, i>r, !n other words, between the edge of the lens and retina proper, early becomes thrown into folds, which give rise to the

nf ihlrtwn D»ya. E^ Epi

u; 2.:. zrnuula Zlnoii: pell \ayeT. After Au^luoci,

ciliary prfxvsnes, and in the adult it persists as the epithelial pi^ient covering of the ciliary processes.

It must be expressly istatwd that the usual description of the develojtment of the iris bj' a growth (>f the optic cup over the lens is erroneous; the walls of the cup exjiand away from the pupil; were the usual d^>sc^iption correct the ])upil of the embryonic eye would have to be larger than the iris J the adult; in other words, larger than is the whole embryo, when the pupil is first developed.

Lens. — Th<; lens is developed from the ectoderm, which comes in contact with the outgniwiug optic vesicles ; the distal wall of each vesicle bcf-oiiiing closely united with a nearly circular area of the epidermis at the side of the head. The attached epidermal area t'lickens and forms the anlage of the lens. Fig. 412, L, while the attfiched wall of the optic vesicle also thickens and forms the anlage of thn retina, It. It is intei-esting to note that the karyokinetio figuri's in the lens anlage are toward the outer surface, and those of the I'etiiml anlage toward the cavity of the optic vesicle, and are there |iiit" pusaiWi- ihat the pupiUftr oriflce enlarges

fnre in homoIogotiR situations in both ectodennal layers. The lens areii now bocomes invsginated. Fig. 412, and may easily be seen in rabbit embryos of the eleventh day, a chick of two davB, or the humiin embryo of the fourth week, as a small pit at the side of the head. To such an extent does the involution of superficial ectoderm take place, that the front or retinal wall of the optic vesicle, as stated in the ])receding section, is pushed close up to the bind or pigment wall, and the cavity of the vesicle is almost obliterated, Fig. 409. Meanwhile, as the pit deepens, its mouth closes over, and the pit becomes a completely closed sac, which at once breaks away from the (iverlying epidermis, which forms a continuous layer in front of it, all traces of the original opening being lost. The closed sac is the lens; it occupies the secondarj- optic cup, Fig. 409, and later < when the cup expands the lens closes the mouth of the cup. Fig. 413. At this stage the lens, L, is a rounded, somewhat flatten^ vesicle with thick walls, and is a strictly ectodermal organ. The space between the retina, R, and the lens, L, gradually increases to form the posterior chamber of the eye, which is occupied by the vitrc")us humor, p. 723, In the chick the sei«iration of lens and retina takes j)lace l>efore, in mammals after, the differentiation of the walls of the vesicular lens has b^un, compare Fig. 4iW with Fig. 413.

The next step is the thinning of the outer or anterior wall of the lens, and the great thickening of its posterior or inner wall. The thickening of the inner wall is rapid, and soon obliterates the original cavity of the lens. This cavity is filled in binls with fluid, b»it in mammals contains scattered cells, which break down jmd disappear a-s the cavity closes; these cells, I think, are probably part of the epitrichial layer of the epidermis. The minute structure of the walls of the lens in its vesicular stage is not known, but it is probably an epithelium of cylinder cells, every cell stretching through the entire thickness of the wall, but the nuclei are scattered at various levels. The anterior wall gradually thins out and is converted into a simple thin layer of cuboidal cells with round nuclei, and is known in descriptive anatomy lis the epithelium of the lens. The posterior wall, Fig. 40'J, thickens rapidly by the growth of its cells, which elongate enormously, without, however, increasing much in thickness, tlms being metamorphosed into the so-called fibres of the lens. The nuclei of the fibres tend to occupy a middle position, hence there is a band of nuclei across the middle of the thickened wall, as shown in Fig. 411. The lens fibre is merely an elongated epithelial cell, and as such it may be readily recognized in the adult. The fibres change their composition so as to be better fitted for the optical functions of the lens than protoplasmatic cells would be, but how the protoplasm of the cells is metamorphosed is unknown. The fibres all stret<»h through the whole thickness of the wall, but become bent so as to form three well-defined systems of curves, so arranged at birth that the systems on the front of the lens alternate in the direction of the fibres with those on the back of the lens, see O. Hertwig's diagram ('* Entwickelungsges.," 3te Aufl., Fig. ^rxS). At the edge of the lens the anterior epithelium is continuous. Fig. 411, with the thickened posterior wall or layer of lens fibres, and there is a grailual transition between the two.

The growth of the lens is, of course, largely due to the growth of the fibres, brt it is supposed that cells are added at the edges of the lens from the anterior to the posterior wall, and converteil into fibres, thus adding new fibres. So far as known, there is no proliferation of the fibres themselves. About two-thirds of the total growth of the lens is accomplished before birth. Buschke is stated to have found the average weight of the lens to be, at birth, 123 milligrammes, in the adult 190.

Capsule of the Lens

Around the lens of the adult is found an anhistic membrane, known as the capsule of the lens. The membrane is presumably homologous with the anhistic layer found under the ectoderm elsewhere and which is permanent in the amnion, p. 334. We have little knowledge of the history of the capsule of thd lens in the embr^'o, except that it grows in thickness and contains no cells. Kolliicer (** Entwiekelungsges.," 2te Aufl., 030) regards the capsule as the produ(*t of the lens, but it more usually is regarded as a specialized part of the matrix of the surrounding mesenchyma.

Tunica Vasoulosa Lentis.* — The lens early becomes suiTounded b}^ a special mesenchymal membrane richly vascularized by branches of the artoria centralis, p. 712, Fig. 410, which reaches the lens from behind, and by branches of the arterial circulus iridis, which reach the lens about its equator. As the lens, being an epithelial structure, contains no vessels itself, its rapid growth on the embryo is probably dependent on supj^ly from the tunica vasculosa. The vessels radiate out from the central artery over the inner wall of the lens, and, branching as they go, pass around the edge of the lena and branch in loops on the anterior surface (see KcUliker, " Entwickelungsgesch.," 2te Aufl.,p. (>50). The network is particularly line and close about the equator of the lens (O. Strhultze) ; it will U» remembered that it is principally at the etiuator that the gn>wth of the lens is supposed to take place. The veins are small vessels which pass off in more or less radial directions from the edge of the lens and join the venae vorticosae of the choroid coat. Until C). Schultze's investigations the veins were practically unknown. The tiuiica vasculosa also extends across the pupil, but toward the close of fo?tal life the vessels abort under the pupil, which thereafter is lK)rdere<l bv characteristic vascular loops (see Kollihcr, "Entwickelungs^es.," 2te Auli., Fig. 409).

In descriptive anatomy three names are employed, each for a part of the tunica vasculosa; at the back of the lens it is the viembrana cap.sularis; at the front of the lens in the centre the viembraiHi jytipillaris, and around the centre (/. e., lK>neath the iris) the membrana capsulo-pupillaris (cf, Kolliker, /.<*., p. iWJ). All these names ought to be discarded. If the membrana pupillaris i>ersists there results atresia pupilke congenita. The pupillary membrane is wanting in birds (Angelucci, 81.1, 150).

The tunica attiiins its greatest development in man during the seventh month and usually disapj>ears before birth, but the time of its disapi^earauce seems to l)e variable.

Optic Nerve

The hollow optic stalk develops into the optic nerve, first by becoming solid, second by acquiring nerve-fibres. It bec*omes solid by the growth of its own walls and the gradual obliteration of its cavity thereby. It acquires nerve-fibres from the thalamencephalon and from the retina, the former set of fibres growing centrifugally, the latter centripetally. Formerly it was assumed that the optic nerve-fibres arose in loco from the cells of the nerves (see for example Hiltner, 86. 1), but there have lKH»n no actual ol)servations to support the assumption. It is iK)Ssi})le that the nerve, l)eing part of the medullary tul)e, develops neurobhists, but it is certain that most of the fibres, if not all, come from the brain and retina, the largest contingent from the brain. Falchi, 87.2, searched for neuroblasts in the opti(^ nerve of cow embryos, but found none.

The choroid fissure permits the wall of the optic stalk to remain dire<rtly continuous with the retina, as already exjJained. The optic stalk consists of a l>asal or inner part, and an outer or distal jwrt, along which latter alone the choroid fissure extends. Fig. 414 repres<»nts a transverse section of the optic nerve as obtainc'd from a sagittfd section of a ra})btt embryo of thirteen days. In the fissure, as des<Til)e<l, p. 712, the arteria centralis retime is dcvelo])ed. The Icjngth of the choroid fissure varies, it being longer in mammals than in birds, and longer in man than in cert*iin jther mammals. Throughout the length of the optic stalk the c(»ntral cavity is obliterates! ; the obliteration begins next the? brain and progr€»ss(^s toward the retina; it is completed in the chick by the seventh day (Mihalkovics, 77.1, 7l0, in man prol)ably by the third month; in man the closure Ijegins during the seventh week (W. His, 89.4, 090). In the distal part of the stalk the choruiil fisanre also becomes closed, but inucli hiter than the central cavity. By these changes the hollow stalk is converted into a solid cylindrical cord continuous with the retina.

The tissue of the stalk, while its cavity is disappearing, changes intoneurogha; in the chick during theiifth day (Mihalkovic^. 77.1, 7lt) appears a clearer layer round the outside, with nerve-6bres in it ; this layer is perhaps homolt^fo is with the Ran Ischleier p ri3, of the central nervous system \fter tl o nerve 1 as become solid Kolliker (" Gr indribs teAufl » ) hnds the following stnicture: The cells are placed ratlially and form a delicate network, the meshes of which are extended longitudinally, and contain numerous bundles of fine nerve-fibres ; and there are also cells arranged in longitudinal rows, which share with the radial cells in forming the network. The nuclei of the radial cells in cow einbr\-oa are oval and nucleolated {Falchi, 87.2).

The nerve-tibres of the opticus begin to appear in the chick the fifth (lay. Mihalkovics and KoUiker have shown that fibres arise in the wall of the thalamencephalon and grow in a bundle toward the median ventral lino following the tractus opticus, p. OSS, As each bundle continues to gn>w in its original direction they cross one another, and each enters the nerve of the opposite side and grows along it towanl the retina; the crossing of the fibres constitutes the optic ckiasma; from the mode of development it is evident that all the fibres from one side must croMS to the nerve of the opposite side. The progress of the centrifugal fibres has yet to be accurately followed. The retina (p. 719) contains in the embryo true neuroblasts, which send off centripetal fibres through the optic nerve to the brain, Froriep, 91.1, has observed in an embryo of Tori)edo ocellata. in Balfour's stage M, retinal neuroblsists, sending fibres into the optic nerve about one-sixth of the length of the nerve towanl the brain and before any other !lt■r^'o- fibres are jiresent. This observation raises the question whether or not the centripetal fibres are developed in other vertebrates also, before the centrifugal. The origin of optic nerve-fibres from the retina was, I believe, first suggested by W. Miiller, 74.1, 37; the suggestion has been confirmed by the observations of His, 90.1, on mammals, and by those of Keibel, 89.1, on reptiles.

Bernheimer, 89. 1, has studied the progressive development of the sheaths of the optic nerv^e-fibres, and reached interesting conclusions by this means as to the course of the fibres.

The optic nerve enlarges both in length and diameter, its enlargement being due to the multiplication of its cells and the growth of its nerve-fibres. It is probably owing to its enlargement that the neighboring mesenchyma around becomes condensed and forms a connective-tissue envelope around the nerve. Concerning the histogenesis of this envelope we know only that it becomes differentiated into two layers — an inner highly vascular layer continuous on the one hand with the pia mater of the brain, on the other with the choroid of the eye, and an outer fibrillar layer continuous with the dura mater of the brain and the sclera of the eye.

Retina

If we consider the structure of the retina, compared with that of the embr^^onic brain, I think the same three primary layers can be recognized as in the dorsal and ventral zones of the central nervous system, see p. 61G. Next to the pigment layer is the membrana limitans externa, which is the boundary of the retinal layer proper toward the brain cavity, which in the eye is represented by the fissure between the pigment layer and the retina proper, Fig. 400, /?. The projection of the rods and cones across this fissure and into the pigment layer is secondary, as explained below. The limitans externa is, therefore, the homologue of the limitans interna of the brain and spinal cord. The cells with their nuclei next this membrane correspond to the inner neuroglia layer, which in the spinal cord becomes the lining epithelium, in the brain the inner white matter plus the ependyma, and in the retina the outer nuclear (or granular) layer and perhaps, also, the inner reticular layer and the inner nuclear layer. These layers of the retina might, therefore, collectively be called the ependymal layer. The nerve-fibre layer is to be homologized with the Randschleier (p. 013). The middle part of the retina between the inner nuclear layer and the internal nerve-fibre layer is comparable to the mantle layer or graymatter layer of the medullary wall ; it includes the inner reticular (or molecular) layer and the ganglion-cell layer of descriptive anatomy. The homologies drawn are probably correct, but they can he definitely accepted only if verified by a fuller knowledge of the development of the retina than we possess at present.

The first step in the histogenesis of the retina proper is the differentiation of the narrow inner zone (i.e., toward the lens), which contains no nuclei, and a wide outer zone (i.e., toward the pigment layer) with numerous nuclei in many layers ; this stage may be seen in rabbit embryos of 4-5 mm. (Lowe, 78.1, 00'^). The narrow zone I identify with the Randschleier (p. 613) of the spinal cord, and the wider nucleated zone with the mantle and inner neuroglia layer of the axial medullary tube.

The second step is the subdivision of the wide nucleated zone into two layers of about equal thickness and distinguished by the character of their nuclei ; the nuclei of the onter are smaller, more oval, and stain more deeply than those of the inner layer. This stage may l«^ observed in a rabbit embrjo of 20 mm. or human of 3«mm; it han been described and figured crudely by Lowe, 78. 1, 604, and accurately by Falchi, 87.2. I interpret the two layers as representing respectively the inner neuroglia layer (ependymal layer) iinil the mantle layer (gray matter) of the braiu. The outer layer with smaller nuclei is to be regarde*! as the anlage of the outer nuclear layer, and later produces the rods and rones. The inner layer undergcx^s further nwHlification.

The third »t<'p is the difTerentiation, 1, of the inner layer (just described and homologized with the mantle layer) into two distinct layers: the inner reticular layer and the ganglion-cell layer; 2, of the oiiter layer (just described and liomlogized with the inner neur<^lia layer of the brain) into three distinct layers: the outer nuclear layer, tho outer reticular layer, and the inner nuclear layer. The five layers are jMirtially distinct in the rabbit at birth, although their differt^ntiation is then still far from complete*!, but are clearly marked out in a human embryoof 915 mm. (Falchi, 87.2, Fig. 3, p. :J87-38!t). Tho fourth step is the development of the rods and cones, which was Bui)erbly investigated by Stax Schultze in Ifliii, 66. 1, 2.3(i-247. Until qnito an advanced i>eriod the niembrana limitans externa of the retinal layer proper remains smotitli. There then appear numerous small projections over the surface of tho inembrana limitaiiB ; the projections are roundeil in form, and are of two sizes. Fig. 415; the ^^^j^-^—^rrv '-■-r Ifl'^'" ones are the anlages of the nxls, the smaller ones of the cones, the latter being the most numerous in the chick. ■riT^. ^-T^v ^ r> ^ Tl e oung rods and cones are at first hemi■^ y^ .-> spl er cal in shape, and each in an outgrowtli ^■-«'*' of elongated sense-cell, the nucleus of

The cells in the retina become differentiated into two main classes, nerve-cell and supporting or neuroglia cells. With tlio former I include the cells wliich abut permanently against the mcmbrana limitatis externa, and which bj' producing tho rucU and cones iKtcome the sensory cells of the adult. As to the exact series of changes thi-ough which the cells puss, our information is scanty. Tho sencR of hiBt<^netic changes do not progress uniformly throughout the retinii, but are more rapid toward tho (jptic iKTve, less rapid toward the It m, or, better said, toward the ciliary l)ody.

Tlio retina proi)er grows more rapidly than the remaining juirts of the eye, and theref<)re is thrown into folds. The folds begin to appear in the human embryo during the thinl month. Ai'cording toKolliker("Grundris8,"2to Aufl., ai'C) the tirst fold arises Iwlowthe entrance ()f tho optic rer%'o and numerous other folils are addetl later. Towanl the end of fcetal life all the fold» gradually disappear, and at l)irth the retina is again smooth.

The macula lutea is ilevel<>[)e(l after birth.

Pigment Layer

The outer lamina of tho secondary opti<; cup, Fig. 41 '2, /*, very early becomes a simple cuboidal epithelium; pigment granules develop in this layer in tho rabbit about the thirteenth day. Fig. 40!i. The pigment4)d epithelium comes to lie close against the limitans externa of tho retinal layer proi»er. When the ro<ls and cones develop they griiw into the layer and become, as it were, burie<1 in pigment ; the pigmenteil epithelium becomes thicker as tho rods and cones l^ecome longer, and remains throughout lifeadiKtinctly epithelial membrane. Its function is HUpposf-d to Ik; to optically isolate the rods and cones from one another.

Blo<>d-Vessels of the Retina. — The following jmragraph is based on (>. Schnltze's n^Imirable memoir on the blood-voHKels of the fcctal eve ("' Festschrift

zum Sojalir. Doktorjubi Idum von KoUiker," lft!i2), where references to the previous observations may be found. A layer of mesenchymal cells is developed quite late (pig embrro of !"0 mm., in man after the third month) over the siirface of the retina toward the vitreous humor; the cells arrange themselves in a verj- distinct network and are then hollowed out to form blood-vessels. The vascularization begins next the optic ner%-e and spreads toward the lens, with the result of forming a layer of vessels (membra na vasculosa retime) which may be injected, and then presents a highly characteristic appearance. Fig, 41ti. Red blood plastiils develop in the network; the vessels were observed in a pig embryo of 175 mm. to have grown from the membrane into the retina. The network is not connected with the arteria centralis retinae, but with vessels which enter around the periphery of the optic nerve.

Fig. 41«. -InJfFMd tbe El's of B f' ' It. velD. Afwr

Lenticular Zone

The term is defined p. 7U. The lenticuhir zone of the secondary optic cup forms the parts beyond the ora serrata, viz., the ciliary processes and the uvea. The opening of the ()j)tic cup is the pupil, Fig. 413, and in early stages is just fiUed by the lens ; at the stage of Fig. 412, the two layers of the optic cup are essentially uniform in character throughout their extent; later, while the optic cup and lens are enlarging, the character of the walls of the optic cup changes, and in a circular zone around the pupil both the pigment layer and the retinal layer of the cup become simple cuboidal epithelium; the thin- walled portion of the optic cup is what 1 have named the lenticular zone, vf. Fig. 411. The pigment layer of the zone very early mjquires pigment granules (in the rab])it by the thirteenth day) and thereafter changes but little histologically. The retinal layer begins to thin out in cow embryos of about 30 mm., in rabbit embr^^os about the sixteenth day, and it quite slowly assumes the form of a cuboidal epithelium. The lenticular zone increases in width, l)iit of its rate of growth I find no record publishoil ; as it becomes wider, we see that one jK)rtion of it overlies the lens, though separated from the lens by the tunica vasculosa lentis; and another portion, which adjoins the true retina, does not rest on the lens. The portion overlying the lens is the anlage of the uvea of the iris, Fig. 417, Uv: the other portion is the anlage of the ciliary processes. The two epithelia of the lenticular zone become closely adherent to one another, and in their further development act as if constituting one layer.

The uvea is the name usually applied to the lenticular zone in the adult, and may be defined as the double epithelial layer covering the choroid processes and the inner surface of the iris. Fig. 417, Vi\

The choroid processes. Fig. 417, arise toward the end of the second, month, or early in the third, as folds of the uvea around the edge of the lens ; the folds are filled with vascular mesenchyma and gradually increase in height ; they are well developed by the fourth month ; in the fifth month KoUiker (" Entwickelungsges.," 2te AuH., 080) found the processes 0.1 2-0. 18 mm. high and 0. KM). 12 mm. wide. The pigment of the uvea is much darker in the embryo over the choroid processes than elsewhere.

Sclera and Choroid

The primitive eyeball consists of the optic cup and lens, and, as it expands, the connective tissue aroiui<l it l^ecomes condensed, forming a mesenchymal envelope, out of which the sclera and choroid coats are gradually evolved. The sclera (sclerotic croat) may be homologized with the dura mater, the choroid with the pia mater.

The sclera is develoi)ed from the outer part of the mesench^Tnjil envelope, and is thickeneil by accretions from the surrounding mesoderm as the eye enlarges ; during foetal life the scleni has no definite external boundary and is comparatively thin ; at what i)eriod the connective-tissue fibrillfe in it begin to develop I do not know.

The choroid or vascular layer is (leveloped from the inner part of the mesenchymal envelope, and, indeed, may be said to begrin before condensation of the connective tissue has begun around the eye, because a capillary network appears very early, making a special vascular layer over the pigment layer of the optic cup — this stage may be seen in a cow embryo of 23 mm. (KoUiker, "Entwickelungsges.," 2te Aufl., Fig. 422). This primitive vascular tunic is continuous with the tunica vasculosa lentis, p. 716. Concerning the histogenesis of the choroid, I have found no satisfactory observations.

Vitreous Humor

By this name is designated the mesenchymal tissue which fills the cavity of the optic cup between the retina and the lens. The tissue appears very early, growing into the optic cup through the choroid fissure, and accompanying the blood-vessels, which form the vascular tunic of the retina and lens ; the tissue at first contains typical anastomosing mesenchymal cells with a large amount of basal substance between them (rabbit of thirteen days). Keibel affirms, 86.1, that no mesenchyma except the blood-vessels grows in, but my sections show conclusively that, as concerns rabbit embryos, he is in error. In the chick, on the other hand, the cells and blood-vessels are both absent (Angelucci, 81.1).

As to the histogenesis of the vitreous humor our knowledge is very unsatisfactory. It probably consists principally in an enormous development of the basal substance, with, perhaps, ultimate abortion of the mesenchymal cells. The space originally occupied by the stem of the central artery persists and is called the hyaloid canaL Over the surface of the vitreous humor is develoj^ed a homogeneous layer without cells, known as the hyaloid membrane, which, therefore, covers the retina, the ciliary processes, and the lens. In the ciliary region it becomes so much thickened that the processes are, so to speak, entirely imbedded in it. The thickened hyaloid membrane of the ciliary region constitutes the suspensoi^y ligament {zonula Zinnii) of the lens; it differs from other parts of the membrane in that it develops radiating connective-tissue fibrils. The fibrils (Angelucci, 81.1, 157) appear in the chick alx)ut the ninth day and in cow embryos of about 90 mm. ; the number of fibrils is at first small, but increases afterward verv much. A hvaloid membrane is also developed over the outer or anterior surface of the lens and is continuous with tho suspensory ligament. The lens is thus completely covered b}- a hyaloid layei', which is known in the adult as the capsule of the lens.

The wandering cells, which are found in the adult vitreous humor, are at first not present, but immigrate later — when, I do not know — although they can be distinguislied in quite early stages. They take, of course, no share in the production of the blood-vessels.

Anterior Mesenchyma of the Eye

The lens at first lies close against the epidermis. Later the mesenchjTna grows in between and forms a layer of some thickness; a cavity (anterior clmmter of the eye) which is at first fissure-like appears in the mesenchymal layer, and divides it into an inner, thinner sheet next the lens, and an outer, thicker sheet next the epidermis ; the inner sheet includes part of the tunica vasculosa of the lens and the connective tissue of the iris; the outer sheet the connective tissue of the cornea. The cells around the cavity assume an epithelial character, epithelium of the anterior chamber, which covers the outer surface of the iris and the inner surface of the cornea.

The ingrowth of the anterior mesenchyma begins in the chick during the fourth day, in the rabbit the fourteenth day ; that is to say, not until the thickening of the posterior wall of the lens is well advanced. According to Kessler, 77.1, a homogeneous layer is formed between the lens and epidermis before the cells penetrate there; ho names the homogeneous layer cornea propria, and considers it a product of the epithelium, but KoUiker (** Entwickelungsges.," 2teAufl., GOO) points out that it is more naturally to be regarded as mesodermal basal substance. The cells of the neighboring mesenchyma gradually make their w^ay into the homogeneous layer and form at first (chick, sixth day) a single laj^er between the lens and ectoderm ; thereafter the number of layers of cells gradually increases. Meanwhile the branches of the arteria centralis retinae spread out and pass on to the anterior surface of the lens, thus converting the innermost part of the mesoderm in front of the lens into the anterior portion of the tunica vasculosa of the lens. The remaining and thicker portion of the mesodermic layer between the lens and epidermis is the anlage of the connective tissue of the cornea.

The next step is the production of the anterior chamber of the eye, which arises as a narrow fissure between the tunica vasculosa and the corneal anlage (Kolliker, ** Entwickelungsgeschichte, *' 2te Aufl., 671). In mammals there appear first (cow embryos 90 mm.) a series of small spaces between the papillary membrane and the cornea proper, and these spaces subsequently fuse into a continuous fissure (Angelucci, 81. 1, 101). I have observed the continuous fissure in a rabbit embryo of sixteen days. It extends at first only to the edge of the pupil, but it soon develops beyond the edge (rabbit eighteen days) imtil it overlies the whole of the uvea; by this means the iris is formed ; the iris is, so to 8i)eak, a circular shelf of mesenchymal tissue bounding the pupil, and itself bounded externally by the cavity of the anterior chamber and covered internally by the uvea, p. 722. Concerning the growth of the anterior chamber we lack precise observations. It is to be regarded as a serous cavity, and the aqueous humor as a serous fluid filling the cavity.

In the chick the tunica vasculosa of the lens does not extend across the pupilla; the first layer of mesenchymal cells which grows in between the lens and epidermis at once forms a thin epithelium (or so-called endothelium), and the space between this layer and the lens becomes the anterior chamber; the layer itself becomes the inner layer of the cornea), Angelucci, 81.1.

Canal of Schlemm

This term is applied to small persistent vessels, Fig. 417, v, on the inner side of the cornea where it joins the iris. Angelucci, 81.1, 103, has observed that these vessels appear early (pig 23 mm.) and persist in birds and mammals throughout life.

Cornea

The cornea consists of two layers : 1, the layer of mesoderm bounding externally the anterior chamber of the eye; 2, the epidermis overlying this area.

The mesoderm is a layer of mesenchyma which increases in thickness and in the number of its cells. The cells next the anterior chamber assume an epithelioid character and finally become a true cuboidal epithelium. The rumainini; cells, which are widely separated by buHal substance, become flattened out; they are commonly termed corneal curpnacle^ in the adult; bundles of connective-tissue fibrils ttitJ developed in the basal substance — just when is uncertain. Against the epithelial lining of the anterior chamber and against the corneal epidennis is developed a hyaloid membrane similar to that formed by the vitreous humor; these membranes have been nametl respectively €ht«tka interna (or membrane of Descemet) and elastiva externa. As neither membrane contains any elastic tissue, both names are to be regretted. KoUiker failed to find either hyaloid membrane of tho cornea in rabbits of twenty days (see His, " Entwickelungsges.," ^te Aufl., 073) and it is prolable that they are both developed late, contrarj- to Kessler's opinion. The corneal mesoderm contains blood-vessels during foetal life and in man, at least, at birth (Kolliker, /.c).

The corneal epithelium (epidermis) develojis, bo far as known, like the epidermis, but its development is arrested at what I have called the amphibian stage, that is to say, there are several layers of cells, but the superficial cells are not flattened out and there is no stratum corneum. KoUiker ("Entwickelungsgesch.," 2to Aufl., (138) hasobserved in rabbit embrj-os that just before the eyelids meet (eighteenth day) the uncovered part of the corneal epithelium is thickened, and that this thickening disappears when the eyelids unite.

Iris and Ciliary Muscle

The iris results from tho extension of the anterior chamber ()f the eye; it may be described as a circular shelf of mesoderm covered on its outer side by the mesodermic epithelium lining the anterior chamber of the eye, and covered on its inner side by the uvea, Fig. 417, Uv. The mesoderm of the iris, 7. is directly continuous with the choroid envelope, tho, of tho eye, and differs in character from the mesenchyma of the cornea and sclera, and it is to be reganled as a prolongation of the choroid layer, cho. The choroid layer. Fig. 417. cAo, thickens considerably as we approach tho oiliarv pwooHsos, /)n), from tlio n^tinal siilo. Tho thit^kiMiiii^^ sulHlivi(io8 into two layors, tho rlioroid pn>|K»r, cAo, ami tho oiliarv hivor, cfl\ rt'L net : the* oiliarv layor is at lii'st aliko in Htruotun* throughout its oxtt^nt, hut vi»ry sikui tho i>Jirt of this biyor ni»art\st tlu* oiliarv priKVssiv* and tho iris ohan^\s in oharaotor, tho tissuo iHH'onios loiwor, tlio (H»11s niovo a[)art, an«i sjmuvs ap|xnir betw«HMi thoni; tho ooUs lon^tlion out, lussiuno a nioro tihrous oharaotor, and ot)nslituto tho liffarnvntiUN pertinatum of anatomy; tho sjMioes oorn^spond to F()nlnNa\s cattals,

The Eyelids arise tpiilo early (oow embryo of *2l\ nun., rabbit of sixtivn tlays) as two folds of tho integument, a little aUne and l)elow tho oornts'U* and they gn>w toward one anotluT until they actually mi^ot and unitt\ Fig. 418. Kaoh fi>ld consists of un<litVonMitiated

mesonohyma and is iM)venHl on Inuh sides by the epidermis. The folds cover not only the cornea pn>iK»r, but also a ivrtain spjico around it; this sp,'uv is the futiin^ roiiJHnctira, As the lids appn^ich one imothor tho epidermis along tho tMlgi^ of each fold thickens (c(^w einbryo of 35 nun.; rabbit embrvo of ninotiHMi days). When tho tnlgi^s of tho folds nuH*t, their epidermal thickenings unite and all trai\^ of any Inumdary disiipjKvirs, as shown bv Donders (l^raofe's Arrhif\ IV., \»'.U) and 'Schweiggt>r-Soidl, 68.1, \*sS; it is said that the lids were ft)rmorly su|.)IhvsihI to Ih> simply adhen^nt, but in reality they actually grt>w ti>gi^thor. The union of the lids t,4ki^ plaiv in !nan early during the t hi ih1 month, in the rabbit tho twentieth day, according ti> Kolliker (** Kntwickelnngsgescii.," '2te Aufl., Tii^S). The \mion i>f tho lids is i>robably inherited fnnn n^ptilian amvsiors, since in certain reptiles the union is |vnnanent. The union i)ersists in man until a short time Ix^fore birth, when the eyoliils ivrmanently so|mrate; tho separation is, 1 think, pn>l>«ibly etfivtiHl by the breaking do^^ni of the i*t*lls in the ivntre of the epithelial layer miiting the eyi»lids. The eyelids do not ojKm in dogs and rabbits U]ilil after birth.

The histological ditYt^riMitiation of the eyelids l>egins after they are solderiHl logt^llior. The epidermis on the outside prcxluces hairs. The i\>ncrt^si\Hl epithelium of the eilges provinces large hairs (e*//fhsht's) ami s<*lvu\\>us glands. The latter develop in a similar manner to the boIwuhhuis gbuulH of tlu^ nkin, p. r)i»\?, but HulwiMpuMitly aiHiuire a largo size and aro known in tlio adult iih tiio Mvihomiati (jlands. The frtH) ends of the oyc^hiHlioH nw inilMMldtHl in tlu* epithelium Iwtween the lids until the eyes omni. The na^HiHierni dnvelopn three layers: an outer, eontinuous with the derniiH of the neighl Miring nkin; an inner, continuous with the eonneiMive tisHue of th«» eonjunctiva; and a middle, in whieh nniw^h^ fihn^H are developed, Kig. 4 IS. As I olwerve the museh^s of the ey(»lids to Imi eontinuous witii the platysnia of the hoiul, it is {irohaliln that they are nKNlifKNitionsof a pjirt of the i)latysma.

Membrana Nictitans (Third eyelid, pli(*a snniilunaris, !Ni(*khaut). — The third eyelid is well dc^vc^loped in birds, (»te., but is rudimentiiry in man. Coneerning its developnunit, nothing neeurat4» is known.

Tear Gland (Lachrymal gland, ThraneinlHise).— The tear gland arises in man during the third month as a solid downgrowth of the epithelium of the eonjunetiva on the nasid side of tint eyeball and close to the up]><T lid, and almost at onc^e forms solid branelieH; the solid anlage afUjrward IxH^nnes hollow (Kolliker, " KntwiekiOungsg(^.," "^te Aurt., ()!♦!»). The fonnation of tla* t<»ar gland bf*gins in tlio c'hick the eighth day (Remak, 60.1, 1*2).

The formation of the lachrymal duct is descTiU-d p. T^HO.

Evolution of the Vertebrate Eye. This subject is, as yet, by no means riix^ for discussion, for we have not only n<i deflnit4f clew to the homologi«?s of the verU'brate eye witli any invurtebrat^j eye, but also no collation of our knowledge of the ey#? suffirirnt to trace the comparative anatomy of the eyo within thn vcrt^<brat4) serif»s. In regard to the evolution of th<» eye within thn vcrt^-braU) series 8e<» W. Miiller, 74.1.

There are two hyiK)thew»s as t^; the origin of the vert#*brate ey#'M : owf, that they are derivcHl from a single m<'dian eye; this is the hy|x>thesis of th<jse who put foremcmt the affinitifH of vert^'brat'eA through Amphioxus with the tunicat^fs, whi^'h are animals with n single meilian eye; the othfr^ that they are derive*! from the iifiin**! eyes of annelids. The first hyjxitheHis has r^Tently found an a/lvocate in Howard Ayers, 90.1, 2*^s, but li#? offers little in su|i|Kirt #>f his opinion Ixjyond his longing to f;st;iblisli a comf>1efe bomolrigv U'tween Amphioxus and true vertebrat^.-s. The wrcond hyfioth<'sis m a corollary of Semper's therir\- that VMrt^rlirat^-s wirre evolv^-d frtftn annelids, and lx?caus** that th^rory has U-com** iriore probable tin our kn<;wlf?<Ige has incTf*awril, it follr*wH that the w?c//n#| hy\Hd)um\9{ has als^i gainr^rl in proliability. For an able, though sjj^^nilative, di^ cu?ision of the way in which the hyfxith^rf^is r:an ^^j work^^j out, s^*? J. von KennelL 91.1.

Anton Dohm'n hyfir>thKicHl sfXr^^njlatioTi.-. 86.2, a.n t/> the phyl/^ g*-n^-sir* of the eye, are not likely, it s^-^jriH to rne, to prove of jx^rrnanent value.

extemus, the tympanum and ear tx>Qes, tui'l the Eustachian tube, to which we may ada the external ear, or so-called coocha. The development of these two parta is very distinct; the membranouc^ labynntli arises as an invaginatiou of the ectoderm ; the ear passages and ossicles arise by modificatioDS of certain of the branchial jirches and clefts of the embrjo; the concha again has an independent development. Accordingly we take up in order the historj- of the otocyRt, of the auditory pass^es and ear bones, and of the external ear.

The membranous labyrinth is developed from a simple otocyst, which is at first a epheruitlal sac of epithelium, and arise-s as an invagination of the ectoderm (epidermis) just over the first viBt-eral or branchial arch. In the history of the labyrinth it is convenient to distinguish the following stages: I, origin of the otocyst; i, first appearance of the recessus labyrinthis vestibuli ; ;t, commencement of the semicirculiir canals; 4, outgrowth of the cochlea; 5, separation of the sacculus from tho vestibule. During all these changes the otocyst or labyrinth is a closed sac or cavity, with a cnntinucnia epithelial lining. Tho process of ditferentiatiou may be considered twofold; 1, the gradual conversion of the simple otocyst into a very complex one; 2, the si>ecializjition of certain areas of the epithelium (maculae ucusticT), connected with the acoustic nerve.

1. Origin of the Otocyst

The ear arises hs a lateral pit, lying somewhat dorsiilly and opposite about the middle of the medulh\ oblongata, and just above the first gillcleft. The pit is an invagination of the outer germ layer (ectoderm) and is at first wide open. This stage has been observed by His in a human embryo of 2. + mm.. Fig. 410, A. In the chick the first sign of the future auditor;' oi^an is a local thickening of the ectoderm usually after thirty hours' incubation; this thickened area is afterward mvaginated and forms the lining of the otocyst In fact the difference between the newly arisen auditory vesicle and the ectoderm in respect of the thickness of the two e))ithelia, is very stnkmg, and the character of the otocyst epithelium is very important, because it exhibits an analc^ry between it and the rudimentary ganglionic sense-organs. As sn^rested by Froriep, there is probably a true serial homology in this case; and tho ear is one of a series of organs extending along the lateral line, none of which, excejtt the ear and nose, persist in mammals save during early einhrj-onic stages, cf. p. 7(1S. In short, the derivation of the complex membranous labyrinth of man from the specialization of one of a long series of general sense-organs in lower ancestral forms is extremely probable.

The mouth of the pit very Hoon cUeeB over, and tlio inva{$ii)Htiun becomes a closed sac, which quickly loses its connection with thu ectoclerm, making a separate spheroidai venicle, tho crtocyrt, Fife- 4 1 tt, B, of. Tlio sac is lined by Ji quite thick epithelium, which contains the nuclei scati«rod at various levels, Fig. 420, and is built up apparently of slender columnar cells, bellied out M'licre each cell contains its nucleus. As yet the oinVnyonic connective tissue (mesodenn) has f urn led no envelope around the epithelium, l>ui liitcr tho cells about the vesicle condense around it and ctjnstituto a sub-epithelial membrane. Tho epithelium retains its cylindrical form over and immediately on the bordersoC all the areas, where the sensorj- hair-cells or socalled auilit<>r>' cfdls are developed; over all the remaining portions it y^ ^^ -H(Tt»«ui ultimately thins out, be- {^""'"^V^L '"-"K comingeitberacubriidal ""™"' '■■'■- >— or a pavement epithelium. In fislies there are Hev<-[i ; in MmijIiibianH, reptitex, and Viinls eight, in man only six, of tli(M<; anaut of ntnmtry cells. It is desirable to call attention X»> this ibinninK out, iMMraiitXf it is usual to find it stated that a thickmiing ariH't), when, in nrality, it is the thinning of ailjacfnt jiurtH wbifji effects tlw diffentiitiation, and though there may l>i; ati abholuti^ tbick'-nirigaWi, yet the thinning n^und al^out in tli<: j/riiicifKil fa/:tt)T.

2. Bececsua Veatibtili.—

which join the ganglion ; compare the history of the olfactory ganglion, |t, H37. The lower portion bood changes its configuration, and in a human embrj-o of about four weeks was found by Kolliker (" Entwickehingsgos.." 2te Aufl.), to have a new rounded protuberance behind and a little outside the base of the recessus, which marks the situation of the future vestibule ; some traces of the Bemieircular canals were alreatly indicated; the lower end of the pear was somewhat elongated preparatory to the outgrowth of the cochlea. The irtocvsts at this time lie near the middle of the hind-brain. Figs, in, D, and 3:J8, ot; Ep iis seen in cross sections,

^ Fig. 4^2, of the head the otocysts are pear-shaped and

lympbatlcus: ok. upper:

■hj,. lioriBHital Bpm utrioiiluB;.S(n-.t<«o
lea. Aft*r W, His,

closely api resawl to the dorsal zones of His of the medulla oblongata. Therecessus^ebt 1 ill rapidly enlarges inditsupper end becomes dilated iij, 4 ( tofcrmtl esaccisendol\mphaticus,S.e, thenaiTower portion develops into the ductus endolj mphaticus of authors. The ductus subsequently becomes greatly elongated, and reaches through the whole pars petrosa, so that the saccus lies within the skull in the dura mater. Kiillifeer (in his "Entwickelungsgeschichte," pp. 744, 745) gives a fragmentary series of measurements of the recessua in mammalian embryos of various ages.

3. The semicircular canals arise next from the walls of the primitive vestibule, and rapidly acquire great prominence, while the coclilea grows out slowly. Each canal first appears as a narrow fold, Fig. :)71, A..SC, P.fic, Ek.M; awide but thin evagination. In the middle of each evagination the opposite walls meet and coalesce, leaving only the rim of the originnl flat pouch; this rim is the permanent semicircular canal. N. Riidinger, 88.2, asserts that each semicircular canal arises from two buds, which elongate as blind tubes, and the tubes uniting fomi a complete canal. The later investigations of R. Krause, 00. 1, and W. His, Jr., 89.1, continnthe older, not Riidinger's view. The canals do not all develop synchronously; the upper vertical canal is first differentiated {R. Krause, 90. 1, 30(1), next the lower vertical c^inal, and last the outer or horizontal canal. W. His found in a human enibrj-o of five weeks that the three evaginations %vere present, but only the two vertical canals had become rings, Fig. 433. The further development of the canal consists in the gradual assumption of the adult form and size, the anipull;e appearing quite early. Fig. 4!.i. Thoembryonic connective tissue about the organ, as a whole, is gradually converted into cartilage and ultimat^y ossifies. The connectne tissue (mesoderm) immetiiately alwnt the otocyst has a diflferent historj which may be readily followe*! in connection with the study of the semicircular canals, and hence may be mentioned now. In Fig. 4^5, the epithelial semicircular canal, in, I, is seen surrounded by a cartilage, f , but sepjirated from it by (I thick layer of gelatinous tissue, (f, and the fibrous perichondrium (future periosteum) , /. Later, the layer, f/, is seiwrated into a thin subepithelial layer, which persists, and a main or gelatinous layer proper, which atrophies, thus leaving the

?^ri-lymphatic8paceabout the canal. , lie gelatinous layer consists of anastomosing connective-tissue cells, b?' ^^" "J^J* ' with, according to Kolliker, a litpiid matrix. The meshes of the network gra lualh incrcise in size, until finally only a few threads are left, thereby establishing the condition in the adult. As tar as known, the whole of tho peri -lymphatic sjMices are formed in this manner, including, of course, the scala tymiiani and the scala vestibuli of the cochlea.

The ainpulhe of the semicircular canals appejir quite early as eiiiai^jements of the canals and develop each a macula acustica, which is stated l»y Kolliker to be found in older embryos covered with a delicate cuticula of considerable thickness, the membrana toctoria of Hasse, the cupula tenninatis of Lang.

4. The cochlea is the third part to grow out from the primordial otocyst, Fig. 4J3, <(7i,- the commencing outgrowth may Ix; olwerved in a human endiryo of five weeks, a sheep emhrj-o of Hi mm., pig of liil mm., ral»hit of 10 mm. It arises from the lower end of the vesicle and grows downward, inward, and forward. Fig. 423, as a canal soniewliat fiattenetl in one diameter and therefore oval in transverse section. Tho epithelial rtx^hloar canal lengthens very much, and, as it lengthens, cur\-es more and more. Fig. 425; on its concave upper side appears the commencement of the future ganglion s]iirale, compare p. 04'!. The cochlear canal is the anlage of the ■uriild media of the adult. In the stage of Fig. 423 it closely resembles the condition found in adult monotrcmes, and also the lagena (cochlea) of birds. By further elougation and coiling the canal KraLhially assumeu the final shape of the ecala media. In man there ]s one c<)mplete coil by the eighth week. Fig. 425, and by the twelfth week all the coils are formed. Baginaky, 86,1, has observed that in very young rabbit embryo there are numerous karjokinetic ligures in the walls of the cochlear canal; later {embryos of bO-CiO mm,) they can no longer be found; they continue longest at the apex ; these observations show that the canal grows throughout its extent and not merely at its apex.

Histogenesis of the Cochlea

Our knowledge of this subject rests principally upon the elaborate researches of Bottcher, 89.1, which have been confirmed and supplementeii bjGottatein, Koiliker ( Entwickelungsgescbichte," 2te Aufl.), Pritchard, 79. 1 , Baginsty, 86.1, and others. The histological development of the cochlea is the same throughout its entire length, but progresses most rapidly at the base, or thestretch nearest the vestibule. I append here the complete history of the cochlea. The first change in the epithelium is in the height of the cell^, those upon the upper side thin out; in other words, that portion of tho epithelium decreases in thickness: it remains a perfectly simple columnar epithelium. Fig. 43G, Ep, and forms the lining of one side of Reissner's membrane, and tho outer wall of the scale. The lower portion of the epithelium which remains thicker fornia the crista, the sulcus, and Corfi's organ. The two divisions of tho epithelium are not sharply separated, but pass gradually into each other.

The second change is that the loss in thickness of the epitheliunj is continued on the imder side, or the wall next the scala tj'mpani, so as to leave two thick epithelial ridges, which are of very unequal dimensions. The larger ridge lies nearest the columella and becomes tho thick lining of the sulcus spirulin. It very early acquires a thick cuticula, the beginning of the «iei«6rnH« tectoria. A very different view is announced by Howard Ayers, 91.2, who states that he has ascertained that the membrana tectoria is really composed of very long hairs, which spring from the cells of tho oi^an of Corti, and that it is, therefore, in no sense a cuticular structure. The smaller ridge lies nearer tho Hgamentum spirale, and is metamorphosed into the organ of Corti, including the supporting cells, the inner and outer hair cells, and Corti's rods. Verj' soon after the two ridges are distinctly formed,;the lamina spiralis begins to grow up between the sulcus or broad inner ridge, and the axis of the cochlea to develop into the crista. The epitheHum on the crista is thus maintained with its upper surface even with that of the sulcus. Over both parts stretches the cuticula. Fig. 42G, 7nt, which grradually thickens into the fully developed tectorial membrane, which has been hitherto supposed to have, at no time, any histogenetic connection with the organ of Corti — compare the reference to Ayers' view above — although it grows out so far as to overhang it. The membrane always remains firmly attaclied to the crista, but is loosely united to the epithelium of the sulcus intomus, and in ■the adult it is probably entirely separated from the sulcus and attached only to the crista. From some unknown cause the lower boundarj- of the epithelium of the crista becomes indistingtiishable. The cflls in the sulcus apparently assume an obliquo position, so that in sections there seem to be several layers of cells. Middendorf and others have been misled to describe a stratified {mekrschithtiges) epithelium in the sulcus.

The small ridge or anlage of the organ of Corti, Fig. 436, 1-7, is made up of four set» of celk ; each set is disposed in a longitudinal row following the spinal curve of the cochlea. The first row, or that nearest the sulcus, sul, is atmposed of a single line of cells, the future inner hair cells. The second row is composed of two lines of cells, 1, 2, the future rods of Corti ; in early stages, as shown in the figure, the cell next the inner hair-cell is considerably larger than ita fellow, but later their relative sizes are reversed (Baginsky, 86.1) ; the third row includes three main lines of cells, 3, 4. 5, the outer hair cells; and the fourth row, (1, has several lines of cells, which become the supporting cells. The further differentiation of the four rows is followed best in transverse sections of the ridge, and in the following description reference is made to the appearance seen in such sections.

tory cells of Corti s organ are viewed from the surface, the hairs are seen to mark out a horseshoe on the top of each cell. The oi>en end of the horseshoe always faces inward, i.e., toward the columella. The base of the cell also acquires one, or, according to Bottcher, two nuclei; the cell becomes fmely granular, and is finally incorporated in Waldeyer's " Komerschicht. Baginsky, 86. 1, 29, maintains, probably rightly, that the two nuclei below the inner hair-cell belong to distinct cells, and are not derived from the iimer hair-cell; he compares them to the socalled Deiter's cells between the bases of the outer hair-cells.

The second and third cells broaden at their bases, where lie their spherical nuclei. The basas widen out rapidly (immediately after birth in dogs) until the two cells form a triangle in section; the width of the base of the triangle exceeds its height. Bottcher, 69. 1, supposed that this triangle was a single cell with two nuclei ; that Bottcher was in error was shown by B. Baginsky, 86.1, 2G. Meanwhile the two nuclei place themselves near the two lower angles of the cells. Next, the cells lose their finely granular appearance and become striated (rabbit embryo of 75 mm.), first along the inner side of the inner r(xl-cell, or next the inner hair-cell ; second, along the outer side of the outer rod-cell, or next the outer hair-cells. The striated lateral portions of the two cells form the two Corti 's rcxis, sensu strict u, A triangular space between the rods and the basement membrane is S(X)n hollowed out, thus forming the tunnel under the arching rods. The protoplasm of the cells is next reduced to small nucleated masses, one at the base of each rod. The further development takes place principally by the growi;h of the rods, until they assume their ultimate shape and size. Recent investigations have added little to the a(»count of the structure of Corti 's rods, given by Waldeyer in Strieker's "Handbuch," 1872, pp. 931-034.

The third band, wliich is three cells wide, Fig. 427, 3, 4, 5, forms the outer hair-cells. Like the other cells, they acquire two nuclei, a larger oval one above, and a smaller one l>elow. This was first observed by Pritchard. The two parts around the two nuclei earl}" become separated into an upper cell (Corti's cells or absteigende Horzellen) and a lower cell (Deiter's cells or aufsteigende Horzellen), 7. The base of the upper cells is at first rounded off, but subsequently a fine process extends down to the membrana basilaris, and the base tapers gradually into the process. The cells become slenderer, and acquire an oblique position about the time of birth. The rod (Stcibchen or Haupthaar) and the horseshoes of hairs (cf. supra) are developed upon the free ends of the cells during the later stages of foetal life. The lower cells taj)er at their upper ends, which are continued each by a fine process. They were supposed by Waldeyer and others to be united in the adult with the upper cells, thus forming twin cells, which have been most fully described by Lavdowsky and Nuel. The development of these twin cells is by no means clearly understood yet. The upper and lower cells appear distinctly separate in new-bom and young animals. The upper cells enlarge at the exiKjnse of the lower. The nucleus becomes smaller and is placed near the top of the cell. The rod (Haupthaar) disappears. The horseshoe of hairs opens toward the Corti's rcxls, as can be best seen in silver preparations. The hairs are more like short rods, vitreous, with rounde<l ends, and are parts of the cell, not of the membrana reticularis. The basal process of the upper cell is inclosed by (Lavdowskj') or fused with (Nuel) the body of the lower cell. The tops of the upper cells (Corti's absteigende, or Stabchenzellen, Lavdowsky) occupy the rings ; the tops of the processes of the lower cell occupy the phalanges of the membrana reticularis. The lower part of the united cells appears as their common body, and contains the low^r nucleus. The nerve-fibre unites with the cell at the side near the lower imcleus. The twin cells end below by a single basal process. The above account is mainly from Lavdowskj\ Nuel agrees with him in the main, but the latter's paper I know only from the abstract in Hoffmann's and Schwalbe's Janresbericht.

Connectetl with the third row, or outer hair-cells, are various structures, which are probably to be grouped under the general head of intercellular formations. Of these the most important are the "Stiitzfasem" (supporting fibres) and the membrana reti'cuh(n\s. The latter is generaUy regarded as the exposed edges of the intercellular substance, the rings and phalanges being the spaces where the free end of the hair-cells are exposed. The " Stiitzfasem" form a network underneath the tunnel, and also a finer network between the outer hair-cells. They were dimly recognized by Bottcher, clearly seen by Nuel, and elaborately described by Lavdowsky.

The fourth row of cells, Fig, 427, G, undergoes no striking differentiation ; it decr(?ases in height from the hair-cells outward, so that the row merges gradually into the low cells of the zona pectin at a, Klein states that in the guinea-pig the supporting cells do not foim, as is usually the case, a simple continuation of the last row of the outer hair-cells, but ride upon the sides of the hair-cells.

Underneath the organ of Corti is developed the membrana basilaris. A large space is developed in the mesenchyma underneath the organ ; this space is the scala tympani, and is apparently a lymplichaml>er. Between the scala tympani and the organ of Corti, there remains a sheet of connective tissue, which contains the vas spirale, Fig. 427, r..v/), and is the anlage of the membrana basilaris. The cells next the epithelium of the scala media flatten out, their nuclei elongate and take radial positions, Fig. 427, ni.bas^ thus marking' out a subepithelial layer from the loose connective tissue below; the looser tissue gradually disappears; the denser subepithelial layer becomes the i)ermanent memorana, in which we can distinguish three layers: a thin homogeneous basement membrane next Corti's organ, a homogeneous nucleated layer, and a lowest fibrillar layer. The spiral vessel underlies the rods of Corti ; the basilar membrane as debcribed is develoixnl only beyond the vessel ; that is, underneath the outer hair-cells. Embryologically si)eaking, the so-called inner zone {hahenula tecta) is not a part of the true tesilar membrane (B. Baginsky, 86.1, ;n-34).

Aoconling to the pre(^eding summary, the cochlea is a tubular extension of the lower side of the ]>rimitive ectodermal otocyst ; upon one side of this tul)0 are two ridges; a larger one, which forms a thick cuticula, the inewbrana iectoria^ and a smaller one which, through complicate<l differentiations, becomes the organ of Corti. The nerves grow to the hair-cells.

6. Saccmus and Utriculus

The separation of the sacculus has been studied principally by Bottcher. There is first developed a constricted tube, the canalis reunienft. Fig. 428, 6, between the base of the cochlea and the central otoc3'stic cavity. Afterward appears a ring-shaped constriction, c/,/, around the main cavity (primitive vestibule, KoUiker), by which it is divided, in most mammals, into two cavities connected by a narrow canal, into which opens the 7'ecessu^ labyrinthi (ductus endolymphaticus of Hasse) ; hence the recessus appears to have two legs, derived from the canal ; one leg loading into the upi)er secondary cavit}', the adult ntriculus, Fig. 423, 428, and the other into the lower cavity, the adult sacculus rotuudus, Fig. 425, 428. These relations, as well as the other essential dispositions of the parts of the labyrinth, are sometimes all recognizable

Fig.

Setti.m tbroujfh the Internal Ear of a in a single fortunatC SCCtioU, aS

A'. K. and K. H, cirtiia^e ; r/., chorda dorsaiis. relations are somewhat different in that the ductus o|^ns directly into the sacculus (Fig. 429, dv) . The developmental pnx»ess resulting in this disposition has not yet been followed out.

The maculce acusticfe of the sacculus and utriculus arise as circumscribed areas where, as before stated, the epithelium remains thick, and is differentiated into auditory cella of elongated forms, with hail's on the free ends.

Of the otoliths the development is unknown. Eolliker merely says, they " appear an quite small puuctiform bodies, and remain a long time in that form, until they finally increase in size and gradually assume a crystaitine form" {"Entwickelungsgeschichte," 1879, p. 7^5).

The development of the definite form of the inner ear is, as we leani from the investigations of Retzius, nearly complete by the end of the sixth month of fcetal life, as shown by the accompanying Fig. ■I'i'.i, which represents the isolated right labyrinth of a six months' htmian embryo, seen from in front and the outside. In the figure the most conspicuous parts are the semicircular canals, the cochlea, and the nerves stained dark by the osmic acid with which the preparution had been treated. The cochlea is a long spiral, commencing with a central blind end, I, and making two and one-half turns, and continuing off tangentially toward the posterior ampulla, ap, to end in a small blind pouch, vb, theVnrhofshUndsack of Retzius. At the base of the pouch springs a small canal, esc, canalis sacculo-cochleiiris {canaiit 7-euniens Henseni), which affords direct communication with the sacculus. In the cochlea (as shown in the figure) we can distinguish the ligajiietitum spirale, l.s, the membrana basilaris^ mb, and the branches, r6, of the cochlear nerve. The three semicircular canals — anterior, ca; external, ce; and posterior, cp — together with their respective ampullse, «a, ae, ap, are easily identified. The anterior and posterior canals have a common stem, ^.v, which leads into the wide vtriculus^ u; from the utriculus a second canal leads into the |)osterior ampulte, ap; finally from the upix^r jx)rtion of the ntriculus arises a wide coecal evagination, reCj the recessus utn'ctih\ the development of which luus not been yet followed out, so far as I am aware. The canalis reuniens, C6r, leads into the saccuhis rotiindus^ which has on one side a large macula acuMica^ mSy and on the other communicates witli the ductus endolymphaticus, de^ of which only the commencement is shown in tlio figure; in reality it ext<?nds clear through the pars i)etrosa, and terminates in the dura mater with a blind enlargement. It is noteworthy that the ductus oj)ens into the sacculus rotvmdus, and not, as in many mammals, into the canal l)etween the sacculus and utriculus. The last-mentione<l canal may be seen in part l)etween the points lettered mn and ?//« in the figure. From this description it is evident that the lahtjrinth is merelu an otocyst of extremehf complex formy and is still a closed epitheliiU sac, continuotis thvomjh all its parts. The acoustic nerve reaches the neighborhocxl of the labyrinth in comi)any with the n. faciiUis, which, of course, passes on beyond. The acoustic nerve divides, first, into two branches: one, the jxisterior, /-(fraud mn, and the other, anterior, (f, which supplies the cochlea and also gives off a few small branches to the macula acustica sacculi, ms, and a more considerable branch, rap, to the posterior ampulla?.

The labyrinth has only six sensorj' areas ; two — namely, the macula neglects and the })apilla acustica lagena* — teing wanting, though present in amphibia, rei)tiles, and })irds. The six present in man are: 1, "2, .*?, in the three ampulhi?; 4, in the recA?ssus utriculi; 5, in the sacculus; (>, in the scixhi media cochlea (the organ of Corti).

The auditory Passages are develoixnl from the first gill-cleft of the embryo. It will l>e remembered, see }). *204, that the clefts are not ojx^n, as in lower vertebrat<?s, but close<l by a tliin membrane. This membrane is the rudiment of the tympanal membrane; the jx^rtion of the gill-cleft within it becomes the Eustachian tube and the cavity of the drum, which are accordingly lined throughout life by an epithelium derived from the ent(xlerm; the inner division of the first gill-cleft has lxH}n named the tfibo-tympanal canal. The portion of the gill-cleft outside the membrane is lineil by ectoderm and becomes the meatus auditorius extern us. That, contrary to the assumption of older writers, the tympanum and external meatus never communicate, even in early stages, was first discovereil by D. Hunt, 79.1. Some recent writers, e. g, Urbantschitsch, 73.1, and N. Kastschenko, 87.1, 0, have maintained that the auditory pfissages are not derived from the first gill-cleft, but they appear to me to offer no justification of this singular view, which has been, in fiu^t, set aside i)y F. P. Mall, 88.1.

In the chick (according to Mall, /. c.) during the third day of incubation an ectixlermal involution is formed from the dorsal i)art of the first (external branchial groove. This involution lies in direct apposition with the doreal part of the first internal hmnohinl ixxtkut, and blends with the facial nerve. During the fifth dtiy of incubiitiou the connection between the facial nerve and the ectwlenn in wvered, and a new outgrowth {canaliii tiibo-tymptmicufi) , from tlw outer i»art of the first internal branchial puciccit, tiikuH itH plat^e. ThiM new outgrowth first extends outward, upward, and forward, hut through the erection of the head its direction is cliangud to outward, upward, mid backward. It forms the tympanic cavity. In niamiiialH tlio development of the tympanal cavity iH <<ssontia]ly t)ie hhuio; it arimit as a blind dilatation of the end of the entiKlonnal jH^rtion of tho tinnt gillcleft. The dilatation at first forms only a thin, Hatteiietl i-avity, wJii<:h for some time, at least in the human embryo, is only |M)tentially proHent, because the opposite epithelial wiUIm grow t4)gethvr and obfitorat« the actual lumen. In the humim embryo at tlireu inontlis the tympanal cavity is still very small. Fig. 4^)6, 7V/, and immediately uvfur

lies the inner end of the solid plug of epithelium, m.ex, rcpreflenting the meatus extemus; immediaU;ly a)x»v'» the tyiiiittinum lies the

malleus, iiml, or uptter end of the cartJlagf; of M<!<;k<;i. The saniu figure shows the auditor}- labyrinth lying in the <-»rti1aginous w;riotic cajisule, the precursor of thew iM;tn>Hum; U;twwn tlw; wx-lilea.

focA, and the semicircular canuls, jS--, lies the primitive vestibule, r, the wall of whiirh cfjmf* <i\»mi: to the outer surfaMj of the ^MtniAia capsule, at a point, /-Of, where the fen(»tmovalis ist^>lMd(;veloued; close to this ixtint aris<.-s the anUge of the hta|xM; it will Ije observed in the figure that then- m a 'naisiderablc H(jw»t ar<*un<l the ear bones between tlu; feu«.-stra ovalis, f.ov, on the one hand.

and the auditory passages, m.e-r, Ty, on the other; this space is filled with embryonic connective tissue. After birth the connective tissue atrophies, while in the same measure the tympanal cavity expands around the bones of the ear (malleus, incus, and stapes), so that these ossicula apparently lie within the tympanal cavity ; but they are, of course, covered by the tjTnpanal epithelium or entoderm, and are, therefore, morphologically owidtde ihe car ity, just as the intestine is outside the peritoneal cavity. As mentioned above, the inner end of the meatus externus, m.ex, lies imme^liately against the t^Tnpanal cavity, Ty; the two passages are separated by a plate composed of two layers of epithelium; this plate is the closing membrane of the gill-clefts, and also the anlage of the tympanal membrane ; mesenchyma is found between the two epithelial layers in the adult, but ' when it penetrates, I do not know. The enlargement of the membane de|)ends chiefly upon the expansion of the tympanum around the malleus, in part also, doubtless, ujxm the actual growth of the membrane; it is said to measure at three months, 2.0 X 1.25 mm. ; at five, 7.0 x 5.5 mm.; at nine, 0.75 X 8.5 mm. (compare KoUiker, " Entwickelungsgeseh. , " 2te Aufl. , 751) .

The inner end of the tul)o-t}Tnpanal canal is transformed into the tuba Eustachii, It becomes small in dituneter, and has a small ofiening into the pharynx just behind the root of the soft piilate. Fig. 323 ; it widens out gradually into the tympanum. Itis lumen is obliterated for a time, presumably, simply by concrescence of the epithelial walls. The cartilage of the tuba appears diu*ing the fourth month, as a plate of hyaline cartilage on the mwlial side of the upper end of the tube (KoUiker) .

The meatus auditorius externus in at first shallow, but gradually deepens, becoming a long horizontal tube; the diameter of this tul>e very early diminishes, and it soon loses its lumen. Fig. 430, by the concrescence of the epithelium; the occlusion continues till after birth. the wax-glands appear during the fifth month, and are develoi)ed, according to KoUiker, after the type of the sweat-glands. A special bone arises, as the so-called annulus t ympanicus^ RToxuid the margin of the tympanum, and subsequently extends itself outward around the meatus; the ring, however, is incomplete on the lower anterior side, and so remains for several years after birth.

The fenestra rot undus snd the fenestra ovalis are spots where the tissue between the labjrinth and the tympanum is so much reduced that only a thin membrane is left over each spot.

The Bones of the Ear are the malleus, incus, and stapes. The development of the first two is described p. 444.

The stapes (compare also p. 440) develops from the connective tissue near the fenestra ovalis. Staderini's careful observations, 91.1, show that in verj' early stages the external jugular vein runs past the tympanum; inunediately below it lies the facial nerve, between which and the tympanum is situated a small branch (arteria stai>edialis) of the carotid artery; the mesenchyma around this artery becomes condensed (embryos of pig, of 15 mm.) and the condensed tissue is the anlage of the stapes, and subseciuently ossifies, according to H. Rathke, from three centres. The artery atrophies in man, leaving the perforated bone, but persists in many other mammals. Staderini seems to me to settle the debate as to the origin of the stapes, and to show that it is to be regarded as an ossification of the feneutra ovalis, not as a modification either in whole or ID part of the visceral skeleton (mandibular or hyoid cartila^s). This view is confirmed by F. Villy, 90.1, 178, who states that in the trog the stapes is formed independently of the branchial cartilages, " as a chondrification in the capsular membrane closing the fenestra ovalis, at a period when the remainder of the capsule is well developed, and not long before the tadpole begins to assume the frog's form. "

The External Ear

W. His has traced out verj- fully the history of the form of the external ear ("Anat. Menschl. Embryonen," Heft III., 211-221). Before the end of the first month there appears around the external opening of the first gill-cleft a series of six tubercles. Fig. 431, A ; two in front, on the hind edge of the first visceral (or the mandibular) iu-ch; one above the cleft, and three behind it. Similar tubercles have been obser^-ed by G. Schwalbe, 91. 1, in the embryos of birds and reptiles. A little later a vortical furrow appears down the middle of the hyoid arch in such a-way as to mark off a little ridge, Fig. 431, A, c, which joins on to tubercle, 3, and descends behind tubercles 4 and 5. The second stage is reached by the growth of all the parts; the fusion of tubercles 2 and 3 and the growth of the ridge down behind tubercle 5 to become continuous with G. After these clumges, it is not difficult to identify the parts, Fig. 431, B. 1 is the tragicum; 2 and 3, together with the arching ridge, represent the helix; 4 is the anthelix; 5, the anti-tragicum; C, the ttenia lobularis. The deep pit bouoded by 1, 2, 3, 4, and 5 is the fossa angulnris. During the latter part of the second month the ear changes its proportions somewhat, becoming more slender ; tuljercle 2 projects farther backward toward the helix, making the seiwration between it and the tragicum more marked, and also rendering the fossa angularis more irregular.

The third stage begins with the third month. The upper and lx)sterior }>art of the concha arises from the surface of the head and gradually, but rapidly, bends over forward, so as to completely cover the anthelix, B, 4, and the upper portion of the fossa angularis, Fig. 431, c. It is during this stage that in mammals the assumption of the pointed form of the ear commences. For a discussion of the development, significance, and frequency of the pointed form of the ear in man, see G. Schwalbe's admirable papers, 89.1, 91.2. The antiversion lasts only a short period, probably not much over a fortnight. The ear now unfolds and shows the anterior tubercle still more projecting than before. Fig. 431, D, and the upper part of the fossa angularis very much reduced.

The fourth stiige commences with the fourth month. The tuberculum anterior encroaches still more upon the fossa angularis, and reduces the lower part of it also to a fissure, hence the tuberculum, 2, itself almost touches the anthelix, 4, and the anti-tragicum, 5. There now appeiirs a ridge which grows out from the second tubercle and imites it wuth the anthelix. Fig. 431, C, Cr./?, and divides the upper part of the fossa from the lower, which latter becomes the opening of the meatus. Shortly after the first ridge a second apI)ears, which unites the second tubercle with the anti-tragicum, Fig. 431, E, Cr.s. Finally the sixth tubercle becomes pendent and appears distinctly as the tcenia lobularis. These changes are completed by the end of the fifth month. The further development is very gradual and is partly post-natal. Of the two ridges, the first formed is ]:)ermanent, and is the cms or spina helicis, while the second (cms supra'tragicum^ His) becomes nearly obliterated; the sulxlivision of the tragicum, already indicated in Fig. 431, E, 1, becomes more marked; the concha enlarges, and its cavity grows more evident. By these and other subsidiary changes, the adult ear is developed. The differences in the ears of adults are mainly the product of secondary modifications.

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